1. Field of the Invention
The present invention relates to a semiconductor substrate, in particular, one having a dielectrically isolated structure, and to a method of producing the same, as well as to the remodeling of a semiconductor device.
2. Description of the Related Art
A dielectrically isolated type semiconductor device is known, which comprises, as shown in FIG. 1, a silicon monocrystal wafer 1, a silicon oxide layer 2, a silicon monocrystal layer (or wafer) 3, a silicon oxide layer 4, a polysilicon layer 5, an N-type silicon layer 6, an N-type impurity diffusion layer 7, a P-type impurity diffusion layer 8, and an N-type impurity diffusion layer 9.
Some methods of producing a semiconductor substrate used in a dielectrically-isolated type semiconductor device will be described.
FIG. 2 illustrates the melt recrystallization method. In this method, a silicon oxide layer 2 is formed on a silicon monocrystallization wafer 1, and a polycrystal silicon (or amorphous silicon) layer 3 is formed on the silicon oxide layer 2. Thereafter, the surface of the layer 3 is scanned with a laser beam or an electron beam at high speed, so as to subsequently grow the solid-phase of the silicon layer.
FIG. 3 shows the SIMOX (separation by implanted oxygen method. In this method, oxygen ions (.sup.16 O.sup.+ or .sup.32 O.sub.2.sup.+) are implanted into the silicon monocrystal wafer 1, forming a silicon oxide layer 2 therein. Thus, the substrate and the silicon oxide layer are isolated by means of the dielectric layer. Then, the wafer is annealed at high temperature, removing the defects caused in the process of ion implantation. Further, the thickness of the silicon layer formed on the silicon oxide layer 2 can be adjusted by carrying out epitaxial growth.
FIGS. 4A to 4E illustrate some of the methods of producing a wafer by means of adhesion. As shown in FIGS. 4A and 4B, the surface of the silicon monocrystal wafer 1 is oxidized by heat so as to form the silicon oxide layer 2. Then, as is shown in FIGS. 4C and 4D, another silicon monocrystal wafer 3 is adhered to the silicon oxide layer 2, and the surface of the silicon monocrystal wafer 3 is abraded until the thickness thereof becomes appropriate.
However, these methods have the following problems.
In the melt recrystallization method shown in FIG. 2, the silicon layer (activated layer) formed by recrystallization of the polycrystallization silicon (or amorphous silicon) layer 3 has an insufficient crystallizability. Therefore, a wafer including such a silicon layer cannot be used for a highly integrated device.
In the SIMOX method shown in FIG. 3, due to the ion implantation performed, the silicon layer (activated layer) exhibits insufficient crystallizability. Further, when carrying out the epitaxial growth, defects in the wafer affect the silicon layer (activated layer) and deteriorate its crystallizability. Therefore, a wafer including such a silicon layer cannot be used for a highly integrated device.
The methods of producing wafers by means of adhesion shown in FIGS. 4A to 4E are free of the problems inherent in the above two methods. In other words, wafers prepared by these methods (FIGS. 4A to 4E) have activated layers exhibiting crystallizabilities similar to those of regular mirror wafers. However, the silicon monocrystal wafer 3 on the activated layer side is likely to be damaged in certain steps of production such as the trenching step and the LOCOS step. Further, the silicon monocrystalline wafer 1 does not have enough getter ability, and the activated layer has crystallization defects such as dislocation and OSF (Oxidation-induced Stacking Fault) may result in the activated layer. Thus, it is very difficult to achieve stable mass-production of the device.